In one process for the production of phosphorus trichloride (PCl.sub.3), chlorine gas is bubbled through molten phosphorus and product PCl.sub.3 is distilled off as formed, leaving a non-aqueous bottoms residue. Over time, unreacted phosphorus, byproduct phosphorus compounds (such as phosphorus oxychloride, POCl.sub.3), heavy metals (such as arsenic and antimony present with the molten phosphorus), heavy metal compounds, and residual PCl.sub.3 concentrate in the still bottoms residue. This residue must be cleaned out periodically. The disposal of this residue poses a significant environmental hazard because of the high phosphorus and heavy metal content.
A variety of processes have been developed for treatment of heavy metal containing wastes to satisfy environmental concerns. In one such process (Japanese patent publication 51-20485--Kawano et al, 1976), an arsenic containing sludge from a sulfuric acid plant is neutralized with powdered lime and incinerated in the presence of excess lime. The product is a poorly soluble arsenate that can be discarded into the sea.
In another process (U.S. Pat. No. 4,948,516--Fisher et al), arsenic sulfide in the waste from phosphoric acid production is neutralized and dissolved with lime, then oxidized to the water insoluble pentavalent state, and finally solidified by treatment with cement or other binder. The solid mass is said to entrap the arsenic and other heavy metal compounds whereby leaching out upon exposure to ambient moisture is essentially prevented. Leachability was determined by the Toxicity Characteristic Leaching Procedure (TCLP) test.
Other fixing or binding approaches include the use of Portland cement (U.S. Pat. Nos. 4,113,504--Chen and 4,142,912--Young) and vermiculite and cement after neutralization with lime (U.S. Pat. No. 4,113,504--Chen).
Soluble arsenic salt containing waste from the manufacture of herbicides has been converted to insoluble form suitable for landfill disposal by reaction in aqueous medium with sulfuric acid and calcium hydroxide in the presence of ferrous ion sufficient to ensure curing to solid form upon exposure to oxygen gas (U.S. Pat. No. 4,118,243--Sandesara).
Arsenic-containing aqueous media, such as waste water or clean-out wash water from PCl.sub.3 production, have been treated with lime in the presence of phosphorus in order to precipitate the arsenic and phosphorus and to allow safe reuse or disposal of the water. Oxidation, as by reaction with chlorine, may be practiced, preferably before the lime treatment, to convert water soluble, trivalent arsenic and phosphorus salts to water-insoluble, pentavalent form. (U.S. Pat. No. 4,201,667--Liao).
In none of these processes has the non-aqueous still bottoms residue from PCl.sub.3 production been treated. This residue is characterized by a high concentration of heavy metals, for example of the order of 1-7% by weight, in the form of water soluble, lower oxidation state, compounds. It is desirable to economically convert this residue to a form suitable for direct disposal in an environmentally acceptable manner.